Apparatus and method for detecting a perforation on a web

Abstract

A photoelectric perforation sensor for accurately detecting a perforation in a moving web. The sensor comprises two large photodiodes in a bi-cell configuration. The photodiodes lie in the direction of web travel. As the perforation begins to pass between an LED source and the bi-cell, light from the LED illuminates one photodiode but not the other. As the perforation becomes centered over the photodiodes, both photodiodes receive equal amounts of illumination and a signal is generated indicating location of the perforation. The circuit for detecting the perforation includes a first comparator circuit for comparing the output of the photodiodes with each other, a second comparator circuit for comparing the sum of the two outputs with a peak detector to arm the first comparator circuit and to prevent false triggering, and a third comparator circuit which compares the output of the peak detector with a reference voltage to shut down the system when an insufficient amount of light is being received by the photodiodes.

We claim: 
     
       1. An electrophotographic reproduction apparatus comprising: a photoconductive member movable along a path;   means for moving the member along the path;   means for performing an operation upon said apparatus in accordance with a timed sequence;   the member including an indicium for use in timing said sequence;   photoelectric sensing means for locating the indicium;   the sensing means including first and second photosensitive means and light generating means adapted to project a different amount of light onto the photosensitive means in accordance with the relative positioning of the indicium vis-a-vis the photosensitive means, the photosensitive means each providing a signal related to the amount of light falling thereon, the sensing means also including (a). means for comparing the signals from the photosensitive means and when they are simultaneously equal, generating a third signal for performing said operation, and   (b). means for adding together the signals from the photosensitive means and for comparing a fourth signal representing the sum of said signals with a threshold signal and for inhibiting the transmission of the third signal when the fourth signal is les than the threshold signal.     
     
     
       2. The apparatus of claim 1 and wherein the sensing means further includes means for establishing a low light threshold and for inhibiting the transmission of the third signal when the amount of light received by the photosensitive means is below the threshold. 
     
     
       3. A method for timing an operation upon a moving photoconductive member, comprising: moving the member along a path, the member including an indicium for use in timing the operation;   providing a sensing means including first and second photosensitive means and light generating means, the light generating means projecting different amounts of light onto the photosensitive means in accordance with the relative positioning of the indicium vis-a-vis the photosensitive means, the photosensitive means each providing a signal related to the amount of light falling thereon;   comparing the signals from the photosensitive means and when they are simultaneously equal, generating a third signal;   timing the operation upon the member in response to the third signal, and   adding together the signals from the photosensitive means and comparing a signal related to this sum with a threshold signal and inhibiting the operation upon the member when the signal related to the sum is less than the threshold signal.   
     
     
       4. The method of claim 3 and including the step of establishing a low light threshold and inhibiting the operation upon the member when the amount of light used for sensing the indicium is below the low light threshold. 
     
     
       5. A method for locating an indicium on or associated with a moving surface relative to a locating device, the method comprising: moving the surface past the locating device which includes a first and second sensor, each sensitive to the proximity of the indicium to the respective sensor;   generating a signal for each sensor related to the relative proximity of the indicium with respect to each sensor;   comparing the signal for one sensor with that for the other and providing a third signal indicating the indicium is located when the signals from the two sensors are simultaneously equal, and   adding together the respective signals for each sensor; comparing a fourth signal representing this sum with a fifth signal establishing a threshold; and inhibiting the transmission of the third signal when the fourth signal is less than the fifth signal.   
     
     
       6. The method of claim 5 and including the step of comparing a sixth signal related to the fifth signal with a predetermined fixed reference signal and inhibiting the transmission of the third signal when the sixth signal is less than the predetermined fixed reference signal. 
     
     
       7. An apparatus for sensing a perforation on a moving web or the like, comprising: a photosensitive bi-cell;   means for projecting light through the perforation onto the bi-cell, the light projecting means being located so that the web material significantly blocks light from reaching the bi-cell when the web is positioned in the light projection path between the light projecting means and the bi-cell;   the cells in the bi-cell being oriented so that during movement of the perforation through the light projection path, light reaches one cell of the bi-cell before reaching the other cell of the bi-cell;   means for comparing the amount of light reaching each of the cells of the bi-cell and for generating a first signal when the amounts are equal;   wherein the cells of the bi-cell each generate an electrical signal related to the amount of light incident thereon; and including means for summing the respective signals of each cell and comparing a signal with a threshold signal and inhibiting transmission of the first signal if the sum does not surpass the threshold.   
     
     
       8. The apparatus of claim 7 and including means for inhibiting transmission of the first signal where the amount of light projected or received is below a threshold level.